Method for covering particles, especially a battery electrode material particles, and particles obtained with such method and a battery comprising such particle

ABSTRACT

Described here is a powder comprising a plurality of lithium-containing particles having a dry, uniform protective layer, wherein the protective layer of the particles is obtained by a sequential vapor phase reaction or adsorption process. Also described is a battery comprising an anode layer and a cathode layer, wherein the cathode layer comprises lithium metal oxide or a lithium metal phosphate, wherein the metal comprises at least one of Nickel, Manganese, Cobalt, Iron, Titanium, and/or Manganese, wherein the cathode particles have a dry, uniform protective layer, and wherein the anode layer comprises lithium titanium oxide particles.

CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

This application is a Divisional of U.S. application Ser. No.14/625,442, filed Feb. 18, 2015, which is a Divisional of U.S.application Ser. No. 11/955,184, filed Dec. 12, 2007. The contents ofthese applications are incorporated herein by reference in theirentirety.

BACKGROUND OF THE INVENTION Field of the Invention (Technical Field)

The present invention relates to a method for covering particles havinga diameter of maximally 60 μm by means of atomic layer deposition. Thepresent invention also relates to particles obtainable by such method,and a battery containing said particles.

Description of Related Art

Such a method is known from the art. Although hereinafter mainlyreference will be made to particles to be used in a battery, for exampleand preferably lithium containing particles, such as LiMn₂O₄, LiCoO₂ orLiNiO₂ as well as other lithium containing materials, such as LiFePO₄and others, the method can be used for subjecting all kinds of particlesin the said size range by means of atomic layer deposition.

In the art, the use of lithium ion batteries has many advantages overother cathode material containing batteries, especially with respect torechargeable batteries. Compared to nickel-cadmium batteries andnickel-metal-hydride batteries, the output voltage of lithium ionbatteries is higher. Secondly, the energy density is higher, resultingin smaller and lighter batteries. Other advantages of lithium ionbatteries are a low self-discharge, good cycle-life and very lowmaintenance. Drawbacks of lithium ion materials are the relatively highcosts and long charging times, and the fact that the batteries age intime, whether they are being used or not.

During the discharge of the lithium ion batteries, lithium ions aretransferred from the negative electrode side of the battery to thepositive electrode side. Recent research activities have provided newelectrode materials, that provide an improved transport of lithium ions.An example of this material is Li₄Ti₅O₁₂, which is used as a negativeelectrode material having the spinel structure. This material has athree-dimensional structure for lithium intercalation (the insertion oflithium into the crystal lattice). With this material, high charge anddischarge rates are possible. A draw-back of this material is that thepotential at which lithium intercalation occurs is much higher than thatfor negative electrode materials used thus far. As a result, the batterywill have a lower output voltage than was usual for lithium ionbatteries. To compensate for this problem, new positive electrodematerials have been developed with higher potentials than the currentlyused materials. Potential (Possible) new positive electrode materialsare based on LiMn₂O₄ (comprising a 50/50 combination of Mn³⁺and Mn⁴⁺),with possible additives like Mg, Ni, like LiMg_(x)Ni_(0.5−x)Mn_(1.5)O₄,(comprising only Mn⁴⁺) which is also of the spinel-type. The positiveelectrode voltage is 4.7-4.9 V, against Li/Li⁺. Therefore, the batteryoutput voltage for a combination consisting ofLi₄Ti₅O₁₂/LiMg_(x)Ni_(0.5-x)Mn_(1.5)O₄ (comprising Mn⁴⁺, and acombination of Ni²⁺ and Ni⁴⁺) can be 3.2-3.4 V, which still is a veryacceptable value.

Hereafter in this description the negative electrode will be referred to(identified) as the anode and the positive electrode will be referred to(identified) as the cathode.

A problem with the above identified cathode material is the dissolutionof transition metal ions, especially Mn-ions, in the electrolyte. Whenthis occurs, the structure of the material changes and there is asmaller number of positions available for lithium intercalation. Inaddition, the high oxidation ability of Mn⁴⁺-ions may lead to adecomposition of the solvents in the electrolyte. These factors lead toa capacity loss that is independent of the cycling but proceedsprogressively in time. The capacity fading increases with temperature:when Li-ion batteries are stored at temperatures of 60° C., a batterymay lose up to 40% of its capacity in only three months time. Theproblem is more severe for high-voltage materials (e.g. Mn and Fecomprising materials) than for “regular” cathode materials. A specificexample of a Fe-containing cathode material, isLiFe_(x)Ti_(y)Mn_(2-x-y)O₄ wherein 0<y<0.3.

Recently, also research has been performed dedicated to the use ofnano-powders in batteries. These powders have several advantages overthe current cathode or anode materials. Firstly, the surface area perweight increases strongly, leading to enhanced charge transfer (fastercharging). Secondly, the diffusion lengths for Li-ions are very short,which enhances the power performance by increasing the effectivecapacity for lithium storage. Thirdly, the nano-powders are much moreresistant to stresses due to expansion and shrinking duringintercalation and de-intercalation of the lithium ions, which causecrystal fatigue and loss of capacity in regular cathode materials.

An important drawback of nano-materials in batteries is the increaseddissolution of the transition metal ions. This dissolution in theelectrolyte is a surface related problem, and therefore increases veryfast with decreasing particle size.

BRIEF SUMMARY OF THE INVENTION

Therefore, the invention aims at providing a method for protecting thenano-particles from dissolution in the electrolyte.

The invention also aims at providing a method for providing a coating onnano-particles, without influencing the electrochemical properties ofthe particles.

The invention especially aims at providing a coating on lithiumcontaining particles of less than 60 μm.

DETAILED DESCRIPTION OF THE INVENTION

According to the invention, the method for covering said particlescomprises the steps as mentioned in claim 1. By means of this method, avery uniform and as thin as possible layer is obtained on saidnano-particles. According to a preferred embodiment, the method furthercomprises the step of (b) subsequently fluidizing said particles in saidfluidized bed reactor using a second reactant gas comprising a secondreactant for substantially completely covering said particles obtainedin step (a) with a monolayer of said second reactant. Such a method isespecially preferred if a layer should be provided on the nano-particlesthat is a product of two different reactants, that are providedsubsequently to the nano-particles. Such is especially very suitable ifthe first reactant adsorbs on and/or optionally reacts with thenano-particles, and wherein the second reactant adsorbs on and/or reactswith the first layer that is provided on the nano-particles. A preferredembodiment comprises the step adding as said first reactant a componentchosen from any of: a hydroxide providing component, an oxide providingreactant, an alkyl metal providing component, a metal alkanolateproviding component, or the like, and adding as said second reactant areactant that is different from said first reactant and is chosen fromany of: a hydroxide providing component, an oxide providing reactant, analkyl metal providing component, a metal alkanolate providing component,or the like. As a matter of fact, if the nano-particles already comprisean oxide layer or a hydroxide layer, the first step of the methodaccording to the invention may comprise adding a reactant that providesa metal on said nano-particles, preferably an alkyl metal compound or ametal alkanolate compound, such that a monolayer of the reaction productof this metal with the hydroxide or oxide may be obtained. If required,a further suitable reactant may subsequently be added, so as to obtain adry alumina monolayer on said material (or any respective metal oxidelayer, for example a zincoxide monolayer).

Any combination of reactants may be added subsequently duringfluidization of the nano-particles, so as to add a first reactant thatadsorbs to and/or reacts with the surface layer of the nano-particles,wherein the second reactant adsorbs to and/or reacts with the firstlyadded reactant, and one or more further reactants are added insubsequent steps for further adsorption to and/or reaction with saidsecondly added reactant.

All steps wherein different reactants are added, are performedsubsequently. The addition of the first reactant in a carrier gas or asa pure reactant, may be followed by the addition of a second reactant,optionally in a carrier gas or as a pure reactant, and may be performedwithout interruption, and optionally with the intermittent addition of agas that is non-reactive (i.e., inert) to the nano-particles and/or thereactant added previously.

A suitable method may consist of adding a fluidization gas to theparticles in a fluidized bed and injecting, or otherwise adding, saidreactant to the fluidization gas. This is a convenient way to keep thefluidization gas substantially constant and wherein the amount ofreactant can be adjusted precisely.

It is preferred that the method is performed on particles having adiameter of maximally 60 μm. Preferably, the particles have a diameterin the range within 10 nm and 500 nm. More preferably, the diameter ofthe particles is at least 10 nm at maximally 100 nm.

It has shown that a battery containing electrode particles that areprotected by means of a nano-layer that is obtained by a methodaccording to the present invention, has an increased lifetime. Althougha fluidization technique has already been used for atomic layerdeposition on small particles, this method has hitherto not been usedfor nano-particles. Fluidization techniques for such particles are onlyknown for systems where the pressure in the fluidization reactor isreduced. According to the present invention, it has shown that it ispossible to use increased pressures in the fluidization reactor, ofabout atmospheric pressure or above. As a rule, this pressure ismeasured at a position above the fluidized bed.

It was regarded impossible to perform an atomic layer depositiontechnique on nano-particles since, due to the very high contact surfacethe heat production would become too high. However, with the presentinvention this has shown to be no problem at all. On the contrary, thetemperature in the fluidized bed is very homogeneous, probably due tothe intense mixing of the particles in the fluidized bed. As aconsequence, the covering of the nano-particles with the reactant(reactants) is very homogeneous, such that nano-particles are obtainedwith a very homogeneous nano-layer thereon, and as a result of which,the batteries have a very constant quality.

The method according to the state of the art for covering nano-particlesconsisted of using a chemical vapor deposition technique onnano-particles, however, without intense stirring of the nano-particles.As a consequence, the layer covering the nano-particles was veryinhomogeneous and hence, the quality of the batteries obtained therewithalso fluctuated greatly.

According to the state of the art, the dimensions of the nano-particleswere increased, with a consequence that diffusion length of the lithiumions increases, and the charging and discharging time also increased.

The invention will now be further elucidated by means of an example.This example is only intended to provide an explanation of theinvention, and should not be regarded as a limitation to the scope ofprotection.

In the fluidized bed reactor, a vibrator is used. Due to this vibrator,an increased fluidization of the nano-particles is obtained. However,this vibrator is not an obligation.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The accompanying drawings, which are incorporated into and form a partof the specification, illustrate one or more embodiments of the presentinvention and, together with the description, serve to explain theprinciples of the invention. The drawings are only for the purpose ofillustrating one or more preferred embodiments of the invention and arenot to be construed as limiting the invention. In the drawings:

FIG. 1 shows a schematic drawing of the experimental setup for theALD-process (atomic layer deposition). It consists of a 26 mm internaldiameter, 500 mm long glass reactor tube that is filled withLiMg_(0.05)Ni_(0.45)Mn_(1.5)O₄ nano-particles with a diameter of 10-50nm. The reactor is placed on a shaker driven by two vibromotors thatproduce a low amplitude vibration at adjustable frequency to assistfluidization. The fluidizing gas is nitrogen. Each ALD-cycle consists offour process steps:

1. Valve V1 is opened, so that part of the nitrogen is led through abubbler containing the organo-metallic precursor and saturated with itsvapor. This vapor adsorbs on the particle surface;2. When the complete particle surface is covered with the precursor, V1is closed and V2 is opened to flush the tubes with pure nitrogen. Thisprevents (undesired) reactions in the tubes;3. V2 is closed and V3 opened to lead the gas through a bubblercontaining water. The water vapor reacts with the organo-metal on thesurface of the powder;4. V2 is opened again and V3 closed to clear the tubes for the nextcycle.

These steps are repeated until a sufficient number of cycles have beenperformed to achieve the desired thickness of the coating.

The variables that can be adapted in the ALD-process are the number ofcycles, coating material, overall flow, reactant concentration, cycletimes for precursor and water, vibration frequency, reactiontemperature, etc. During the process the temperature, pressuredifference and pressure fluctuations are recorded.

For the experiments described in this paper, only the fluidization partof the set-up has been used, i.e., gas without reactants for thefluidization, assisted by vibration. The gas flow was varied from 0 to21/min. (velocities of 0-63 mm/s), and several vibration frequencieswere used, ranging from 0-47 Hz. Higher gas velocities were not usedbecause the particles started to be elutriated from the column.

Pressure fluctuations were measured at a frequency 20 of 400 Hz usingpiezo-electric pressure transducers, Kistler type 7261, at two heightsin the column: 50 mm and 125 mm above the gas distributor. For titaniaparticles, that were used in some experiments, the data from 125 mm aregiven here because of a blockage of the lower measuring point after sometime; for the cathode particles the height of 50 mm was used due to thelower initial bed height. For the titania particles, the data from thehigher and lower measuring point were comparable. The fluidizationexperiments were done at room temperature and atmospheric pressure.

Two types of particles were used: the first type is theLiMg_(0.05)Ni_(0.45)Mn_(1.5)O₄ cathode material, which was prepared byan auto-ignition method (described by Lafont et al.). FIG. 2 shows TEM(transmission electron microscope) pictures of this material. Theparticle dimensions observed in these TEM images are 20-100 nm. A(Brunauer-Emmett-Teller) BET-analysis rendered a surface of 6.4 m²/g,from which an equivalent diameter of 213 nm can be calculated. Laserdiffraction showed a very wide particle size distribution, ranging from40 nm or smaller (40 nm is the lower limit of the apparatus) to 60 μm(clusters). Combination of these measurements leads to the conclusionthat the particles form clusters, and that part of the clusters are“hard” aggregates, with some necking between the primary particles. Tomake a comparison possible, also a more common type of nano-particleshas been investigated: commercial titania particles. These particleshave a diameter of 20-25 nm and a surface area of 90 m²/g (data frommanufacturer Kerr-McGee Pigments). It is expected that it is a loosepowder and at all aggregates in this powder are soft aggregates thatbreak up easily.

FIG. 3 shows the relative bed expansion during the experiments. Tocalculate this, the minimum bed height as measured during allexperiments, was taken as the initial bed height H₀. This minimum wasfound when the bed was compacted at the highest vibration frequency. Forthese experiments, the vibration frequency was set to a fixed value, andthe gas velocity adjusted. We started with the lowest frequency.

However, the experiments were also carried out with particles with ahistory of vibration, also at high frequencies; these particles aremarked with an * in the figures. The graphs confirm that the initial bedheight depends on the vibration frequency, at higher frequencies theparticles are packed closer.

Visual observations of the fluidization behavior of the cathodeparticles suggest that at low gas velocities, there is some channeling.At higher velocities, the eruptions at the bed surface are more violentand appear to originate from (small) bubbles, although these are hard todistinguish since the powder is black. The vibrations have someinfluence: at high frequencies bubbles start to appear at lower gasvelocities. The effect was not quantified due to aforementionedvisibility problems. For the (white) titania powder it is easier todistinguish channels and bubbles. For each velocity and frequency, thereis a certain part in the bottom of the bed that is not moving. Somelarge aggregates can be distinguished here and channeling occurs inbetween these aggregates. The height of this part decreases with gasvelocity and vibration frequency. Above this bottom zone, the bedfluidizes with small bubbles. A memory effect could be noticed for bothparticle types, although it was stronger and lasted longer for thetitania. The non-moving bottom zone was much smaller for particles witha history of vibration than for “fresh” particles. An explanation couldbe that part of the aggregates was broken up by the high frequencies,and only the very large (hard) aggregates remained. The bed expansionfactor H/HO reached a maximum value of 2.0 for the cathode particles and1.63 for the titania particles. It was also found that when thevibration and gas flow are stopped, the bed does not return to itsinitial height, and even after several days it may still be expanded(H/H01, 4), showing that the aggregates are very loosely packed. Themeasured porosities for the cathode particles were in the range of0.66-0.83, and for the titania it was 0.87-0.92.

FIGS. 4A and 4B show the standard deviation of the pressure signalduring the experiments, which could provide information on the regime inwhich the fluidized bed is operating. The pressure fluctuations in theexperiments with a high vibration frequency are determined mainly by thevibrations, the influence of the gas flow was minor. This was confirmedby the power spectrum, where high peaks occurred at the vibrationfrequency. When there is no vibration or vibration at a low frequency,there is a noticeable influence of the gas flow on the pressurefluctuations, as is observed in a regular gas-fluidized bed as well.

For the cathode particles, the sudden rise followed by a decrease in thefluctuations at high frequencies could indicate a transition frombubbling to turbulent regime. However, more data are necessary toconfirm this and explain the mechanism. For the TiO₂ this transition wasnot observed for the studied range of gas velocities. The data seriesfrom particles with a vibration history show that this history and thechange in fluidization behavior it causes do not have a large influenceon the pressure fluctuations.

FIG. 5 further shows SEM (scanning electron microscope) photographs ofparticles obtained according to the method of the present invention andwherein as a first reactant water was added for forming a hydroxidemonolayer on the particles (LiMn₂O₄). Then, trimethylaluminiurn (TMA)was added to the fluidization gas so as to perform a reaction of saidTMA with the hydroxide monolayer. Subsequently, a further addition ofwater was performed, and a first monolayer of alumina on said particleswas obtained. This combination of steps was repeated until an aluminalayer on said particles was obtained in a sufficient thickness and wasvery homogeneous. The layer turned out to have a thickness of about 2 nmand consisted (by means of EDX (energy dispersive x-ray) in a SEM) ofaluminum oxide.

The FIG. 6 shows the results on cyclic behavior of repeatedly chargingand discharging batteries made from the coated particles according tothe invention and uncoated particles as a reference example. Both at lowtemperature (20° C.) and high temperature (60° C.) the capacity at fastand slow discharge and charge rate is much higher in the batteriescontaining the coated particles as cathode material. Also, the uncoatedparticles show a clear fading in capacity due to degradation of thecathode material.

Therefore, from the above example, it can be concluded that the method,according to the present invention, is a suitable way for providing aprotective layer on nano-particles.

What is claimed:
 1. A powder comprising a plurality oflithium-containing particles having a dry, uniform protective layer,wherein the protective layer of the particles is obtained by asequential vapor phase reaction or adsorption process.
 2. The powder ofclaim 1, which further comprises at least one of Nickel, Manganese,Cobalt, Iron, Titanium, Magnesium or Phosphorous.
 3. The powder of claim1, wherein the particles comprise a lithium metal oxide or a lithiummetal phosphate, wherein the metal comprises at least one of Nickel,Manganese, Cobalt, Iron, Titanium or Manganese.
 4. The powder of claim1, wherein the sequential vapor reaction or adsorption process is atomiclayer deposition.
 5. The powder of claim 1, wherein thelithium-containing particles have a lithium hydroxide surface, whichforms a protective layer comprising lithium metal oxide when reactedwith a metal containing reactant.
 6. The powder of claim 5, wherein themetal of the metal containing reactant comprises aluminum, and theprotective layer comprises lithium aluminum oxide.
 7. The powder ofclaim 1, wherein the particles are adapted for use in an electrode. 8.The powder of claim 1, wherein the lithium-containing particles comprisemanganese having a valence state of at least one of Mn²⁺, Mn³⁺ or Mn⁴⁺.9. The powder of claim 1, wherein the lithium-containing particlescomprise nickel having a valence state of at least one of Ni²⁺, Ni³⁺ orNi⁴⁺.
 10. The powder of claim 1, wherein the lithium-containingparticles are anode particles.
 11. The powder of claim 10, wherein theanode particles are lithium titanium oxide particles and the protectivelayer comprises a metal oxide.
 12. The powder of claim 11, wherein thelithium titanium oxide particles have a specific surface area ofmaximally 90 m²/g.
 13. The powder of claim 1, wherein thelithium-containing particles are cathode particles.
 14. The powder ofclaim 13, wherein the cathode particles have a specific surface area ofmaximally 6.4 m²/g.
 15. The powder of claim 13, wherein the cathodeparticles comprise lithium metal oxide or a lithium metal phosphate,wherein the metal comprises at least one of Nickel, Manganese, Cobalt,Iron, Titanium, and/or Manganese.
 16. The powder of claim 15, whereinthe lithium metal oxide comprises at least one of lithium cobalt oxide,lithium nickel oxide, lithium manganese oxide, or lithium nickelmanganese oxide.
 17. A battery comprising the powder of claim 1, whereinthe lithium-containing particles comprise anode particles and cathodeparticles.
 18. The battery of claim 17, wherein the anode particles arelithium titanium oxide particles and the protective layer comprises ametal oxide, wherein the lithium titanium oxide particles have aspecific surface area of maximally 90 m²/g, wherein the cathodeparticles have a specific surface area of maximally 6.4 m²/g, whereinthe cathode particles comprise lithium metal oxide or a lithium metalphosphate, and wherein the metal comprises at least one of Nickel,Manganese, Cobalt, Iron, Titanium, and/or Manganese.
 19. The battery ofclaim 17, having a battery output voltage of 3.2 to 3.4 Volts.
 20. Abattery comprising an anode layer and a cathode layer, wherein thecathode layer comprises lithium metal oxide or a lithium metalphosphate, wherein the metal comprises at least one of Nickel,Manganese, Cobalt, Iron, Titanium, and/or Manganese, wherein the cathodeparticles have a dry, uniform protective layer, and wherein the anodelayer comprises lithium titanium oxide particles.
 21. The battery ofclaim 20, wherein the lithium metal oxide comprises at least one oflithium manganese oxide or lithium nickel manganese oxide.
 22. A powdercomprising a plurality of lithium-containing particles having a dry,uniform protective layer of up to 2 nm in thickness.